Description: Do the most epic slam dunk ever.
DLC: Origin Of Pain

The description doesn't tell you much about the achievement but to obtain you must do the following:
Choose the Hill Climb skill challenge and pick the phoenix 250cc.

Ride up the first ramp a little bit and allow yourself to roll down. You must be on your front wheel and once your back wheel touches the wooden raft do a backwards bunny hop. The video guide will show you. Stay on the raft and it will start drifting backwards. Once you get under the basketball on a piece of rope press y to bailout and hold it to grab and hold onto the ball.

Angle yourself 45 degrees right and keep going up controlling your rider so he doesn't hit the sides.
At the top lower him into slam dunk position and let go with Y. If done correctly fireworks will go off and the achievement will unlock.

Repeat if failed. Here is a video guide by "Maka91Productions" to help you.